1. Field of Invention
The invention generally relates to electrophotographic photoconductors and manufacturing methods, and particularly to photoconductors and methods principally comprising a conductive substrate and organic photosensitive layer and used in electrophotographic systems.
2. Background Art
Electrophotographic photoconductors generally comprise a photosensitive layer having a photoconductive function laminated on a conductive substrate. Recently, organic photoconductors, which use organic materials as functional components which generate and transport charge, are being actively studied and developed for copier and printer applications. Organic photoconductors have potential benefits in flexibility of material selection, high productivity, and safety.
Photoconductors generally need functions to retain surface charges in the dark, to generate charges upon receiving light, and to transport the generated charges. There are generally two types of photoconductors: a single layer type and a laminated layer type (or a function-separated type). The single layer type of photoconductor comprises a single photosensitive layer that performs all the above-mentioned functions. A photosensitive layer of the laminated layer type of photoconductor is a lamination of two function-separated layers. These include a charge generation layer principally performing the function of generating charges upon receiving light, and a charge transport layer principally performing the function of retaining surface charges in the dark and transporting the charges generated in the charge generation layer upon receiving light.
The photosensitive layer is generally formed by coating a conductive substrate with a coating liquid that is prepared by dissolving or dispersing a charge generation material and a charge transport material together with a resin binder in an organic solvent. The outermost layer of an organic photoconductor frequently comprises a resin binder of polycarbonate, which is resistant to friction between the layer surface and paper or a blade for toner removal, which is mechanically flexible, and which is transparent to the exposing light. Bisphenol Z polycarbonate is widely used as the polycarbonate resin binder. Application technologies using polycarbonate are disclosed, for example, in Japanese Unexamined Patent Application Publication Nos. S61-062040 and S61-105550.
Further, polyarylate is commonly used as a resin binder of the photosensitive layer as disclosed in Japanese Unexamined Patent Application Publication Nos. S55-058223, S56-135844, H10-288845, 2002-148828, and 2002-174920. Studies on polyarylate have been made for improving durability and mechanical strength.
However, photoconductors comprising bisphenol Z polycarbonate resin binder, have a drawback since they are liable to generate solvent cracks or cracks due to a hand contacting on the photosensitive layer. Solvent cracks are apt to occur due to contact with cleaner solvent during cleaning of the photoconductor and the charging member. In contact charging systems in particular, large cracks occur in the photosensitive layer if the photoconductor is subjected to contact with the charging roller after cleaning and before complete volatilization of the cleaning solvent.
Environment-conscious trends are promoting a desire for recycling, and photoconductors and cartridges tend to be usually recharged and cleaned for recycling. Therefore, it has become urgent to solve the problem of solvent cracks. In a liquid development process, there is also a problem of frequent occurrence of solvent cracks because the photoconductor directly makes contact with carrier liquid dispersing toner particles. This problem also strongly needs to be solved.
To cope with this problem, Japanese Unexamined Patent Application Publication No. S61-062040 discloses use of a mixture of bisphenol A type polycarbonate resin and bisphenol Z type polyearbonate resin. Japanese Unexamined Patent Application Publication No. S61-105550 discloses use of a copolymer resin of a bisphenol A structure and a bisphenol Z structure. However, both of these disclosures do not solve the problem (see above).
It has been proposed to form a surface protection layer on the photosensitive layer for the purpose of protecting the photosensitive layer, thereby improving mechanical strength, and improving surface lubricity. Such a surface protection layer also leads to the above-mentioned problem of cracks in the surface of a photosensitive layer.